In recent years the so-called “speed density” control system, which needs to know the mean intake pressure with adequate accuracy in order to calculate the mass of fresh air trapped inside each cylinder, has become increasingly widespread for the control of an internal combustion engine.
A modern internal combustion engine for cars is provided with a number of cylinders (typically four in line), each of which is connected to an intake manifold via two intake valves and to an exhaust manifold via two exhaust valves; the intake manifold receives fresh air (i.e. air arriving from the outside environment) through an intake duct controlled by a butterfly valve and is connected to the cylinders via the respective intake ports, each of which is controlled by the corresponding intake valves. In an internal combustion engine fitted with an intake manifold, the pressure pulses inside the intake manifold are modest due to the effect of the volume of intake manifold itself; in consequence, in order to determine the mean intake pressure in an internal combustion engine fitted with an intake manifold (i.e. the average value of the pressure inside the intake manifold), it is sufficient to measure two intake pressure values via a pressure sensor positioned inside the intake manifold on every engine cycle (i.e. every 720° of rotation of the drive shaft).
Owing to the numerous advantages provided by the “speed density” control system for controlling an internal combustion engine, there is a desire to use this system on internal combustion engines for motorcycles or racing as well; however, internal combustion engines for motorcycles or racing do not normally have an intake manifold and each cylinder is directly connected to the air cleaner box (containing the air filter) via a short intake port (or intake trumpet) controlled by a respective butterfly valve. In this case, a pressure sensor is inserted inside each intake port; however, in an intake port of an internal combustion engine without an intake manifold, pressure pulsing is extremely high, even in idle conditions, and therefore it is much more difficult to be able to calculate a mean intake pressure value with sufficient precision without employing an electronic control unit with very high computing power.